In general, it has been found that most deposits of nickel-cobalt laterites contain three major zones based on morphology, mineralogy, and chemical composition. These three zones from the base to the surface atop weathered parent bedrock materials are the saprolite zone, the transition zone, and the limonite zone with large variations in total thickness of the deposit, as well as individual zone thickness. The saprolite zone consists of three separate subzones: rocky saprolite, saprolite, and ferruginous saprolite. This saprolite zone consists predominantly of "saprolitic serpentine" and a large variety of nickel-magnesium silicate minerals that belong to the septochlorite group of minerals as defined in "An introduction to the Rock Forming Minerals" by Deer, Howie and Zussman; Longman Group Limited, London, UK, 1983. Septochlorites (general chemical formula: A.sub.6 (B.sub.4 O.sub.10)(OH).sub.8 wherein A represents Mg, Fe, Ni, and/or Al and wherein B represents Si, Fe, and/or Al) are characterized by serpentine like layers, with each layer having a tetrahedral (Si Al).sub.2 O.sub.5 component linked to it by a tri-octahedral brucite-type (MgO) compound. Various arrangements of layer stacking are possible and they give the laterite deposits its layered and clayey structure. The weathering process or serpentinization of the ultrabasic bedrock (a low nickel (.about.0.2%) and high (.about.5%) iron containing magnesium olivine mineral) is characterized by a decrease of Mg in the ultrabasic and an increase in Ni and Fe upward. The resulting saprolite zone contains between 0.5 and 4% Ni.
The not-well-defined transition zone is composed essentially of nontronite-type clays (smectite group) and quartz. It also commonly contains Ni in the range from 1.0 to 3.0% with coexisting Co ranging from 0.08% up to 1% Co (associated with asbolane, a hydrated manganese oxide). The limonite zone (with nickel ranging from about 0.5 to 1.8%) consists of an upper hematite-rich section and a lower goethite-rich section and is rich in Fe, Al, and Cr. Sometimes the weathering has not been fully completed and either the hematite or the goethite rich sections are not present. Alternatively, depending upon the climatic condition the limonite zone will still contain residual iron-aluminum silicates, such as chlorite that are nickeliferous.
While heap leaching copper ores is well known as a unit operations, there are several differences between heap leaching of copper containing ores that also contain some clay components and the lateritic ores that have a substantial clay component.
The laterization process occurs mainly in tropical or subtropical environments, where warm, humid, and good drainage environments are prevalent. This process occurs with slow dissolution of olivine and pyroxene along micro-fractures and grain-boundaries of these minerals usually removing soluble metal like magnesium and leaving porous silicate-(serpentine), silica-(chalcedony, tridymite) and iron- skeleton (sieve texture) or box-work or sometimes called mesh texture especially for serpentine.
In tropical wet-dry climates, clay formation is more prevalent due to poor drainage and high pH environment. In addition, it has been reported that in areas where the drainage is poor, a greenish iron-rich silica gel is first formed and recrystallization of the gel occurs over time to form nontronite (Fe-smectite, or greenalite), hectorite (Ni-smectite), and saponite (Mg-smectite). Nontronite mineral profiles are very common in tropical climates with prolonged dry seasons, e.g., Ivory Coast, the Western Australia nickel deposits, Cuba, Brazilian Shield, etc. Amorphous iron-nickel rich silica gel is very common in various West Australian deposits such as the Bulong and the Murrin Murrin nickel deposits. These silica gels are readily, but slowly soluble in acidic solution.
It has been found that the permeability of lateritic ore is largely controlled by the type of mineral occurrence, mineral morphology and particle size. Although the mineralogy of lateritic ore is rather complex and widely variable from deposit to deposit, there is some commonality or similarity of mineral morphology in the worldwide lateritic nickel deposits, e.g. oolites and pissolite of iron oxides in ferricrete horizon, mesh or sieve texture of serpentine, skeleton or sieve texture of chalcedonic quartz, and iron oxides. These morphological structures enhance permeability of solution and preserve physical stability of individual minerals. Although experimental data on behavior of nontronite on permeability is not available, it is expected that nontronite may have properties similar to other smectitic clays.
Nontronite in lateritic ore, however, usually occurs with silica box-works, precursor silica gel and transitional amorphous compounds between gel and nontronite. This mineral morphology of precipitated silica gels is not generally found in porphyry copper ore deposits.
Samples of such a nickel containing ore with a substantial clay component is provided in the following table where rheological properties of a number of different laterite ores are presented.
Solid Yield Bingham Content Strength Viscosity Ore Type (wt %) (dynes/cm.sup.2) (centipoise) West Aus- Limonite/Saprolite 25 722 80 tralian Ore New Cale- Limonite 28 40 11 donian Ore New Cale- Saprolite 28 1700 57 donian Ore New Cale- Blend 28 400 18 donian Ore Indonesian Limonite 30-40 &lt;150 &lt;15 Ore
The high yield stress and viscosity of the ore slurry can result in inadequate agitation in high pressure leaching or atmospheric pressure leaching operations.
In addition, these ores may have the following particle size distribution.
 CUMULATIVE PARTICLE SIZE DISTRIBUTION (WT-% PASSING) NICKEL LIMONITE ORE MICRON NICKEL SAPROLITE ORE 80 37 30 100 297 45 841 50 1680 58 6350 84
The above table clearly demonstrates that both limonite and saprolite nickel ores contain significant quantities of very fine material as well as a significant clay content.
U.S. Pat. No. 5,571,308 describes a process of heap leaching high magnesium containing lateritic ores such as saprolite. The patent points out that the "clay-type" saprolite exhibited poor permeability during filtration. As a solution to this problem, the patent noted that pelletization of the ore is an important expedient to assure uniform distribution of the liquid-reagent throughout the heap and to provide pellets of sufficient shape integrity to resist gravimetric flow and yet assure the desired permeability for irrigation or percolation of the reagent solution throughout the heap. The reported irrigation flow flux was 10 l/hr/m.sup.2, which is at the high end of conventional copper and gold heap leaching practices. Nickel laterites have a rather high acid consumption (hundreds of kgs. of acid per ton of ore versus about ten kgs. per ton of copper ore) because of the soluble magnesium content, low-flux heap leaching could require a long time. For example, heap leaching of nickel laterite ore could require as much as two to five years at this percolation flux to obtain reasonable nickel extraction (&gt;80%). Therefore, the acceleration of nickel leaching kinetics is an important issue for the techno-economic success of heap leaching nickel laterite ore.
Heap leaching of laterites by sulfuric acid at ambient temperatures is reported in Heap Leaching of Poor Laterites by Sulphuric Acid at Ambient Temperatures, S. Agatzini, et al, Hydrometallurgy 1994, Institution of Mining and Metallurgy, London, 1994 page 193-208. There, the laterite ore had the following particle size distribution:
 Particle size (microns) Wt. % +850 80.2 -850 to +360 4.5 -360 to +180 4.8 -180 10.5
Thus, it can be seen that this ore was coarse and hence very competent as it is not fine-grained and therefore, does not contain much clay at all. In addition, the reported ore consisted of nickel chlorite, hematite, quartz, chromite, and small amounts of talc, illite and diaspore. The ore was characterized by a pissolitic texture, with pissolites composed of microcrystalline hematite aggregates set within a peliomorhic red matrix. The matrix consisted primarily of quartz and chromite grains surrounded by chlorite and fine dispersed hematite and secondary illite and diaspore. The competent rock reportedly did not contain any of swelling smectite-type clays, like nontronite, saponite, and hectorite. Moreover, the reported ore contained a significant presence of limestone (5.68% CaO), which generally is never present in an amount larger than a few 100 ppm in a typical nickel laterite. Furthermore, the reported ore did not contain goethite, which is always readily present in typical nickel laterite ores. Thus, this paper does not address the use of heap leaching of a nickel containing ore that contains a substantial clay component.
The present invention teaches how to operate a heap leach to maximize the recovery of nickel at maximum efficiency independent of the recovery procedure for extracting the nickel from the effluent. In particular, the present invention solves the problem of extracting nickel (and cobalt) from those nickel containing laterite ores having a tangible clay component.